What goes on inside your LCD screen.
New displays look great in both sun and shade. Here’s why.
We all spend hours looking at LCDs at home and at work as well as on our boats. But how many of us know how they work or—more importantly—how to choose a good one? Equipment manufacturers bombard us with snippets of information, but what does it all mean? Is a QVGA better or worse than WXGA? Just how big is a nit? And is CCFL on its way in or out?
To start at the beginning, LCD stands for Liquid Crystal Display, a high-tech sandwich with a layer of liquid crystals at its heart. But just as it takes more than beef to make a burger, it takes more than liquid crystals to make a display.
Two particular properties make liquid crystals special. The first one is that they combine the shape-shifting ability of a liquid with the regular molecular structure of a solid and can be persuaded to change from one to the other when an electrical voltage is applied to them. The other one is that they can transmit and change polarized light.
A very simple LCD is made up of seven layers. (See illustration below.) At the back of the screen is a layer of film that polarizes the light from the display backlighting so that all the electromagnetic waves that make up light are going in the same direction. Then comes a glass filter etched with a pattern of ridges and furrows, like a plowed field but reduced to molecular scale. Then comes one of the electrodes that will apply a voltage to the liquid crystal.
Four layers in, we come to the liquid crystal itself, before repeating the original three layers in the opposite order, but with one crucial difference: In the inner layers, the furrows in the filter are lined up with the polarizing film. In the outer layers the film and furrows are also lined up with each other but are rotated 90 degrees from those in the inner layer.
Any experienced boater has undoubtedly owned a pair of polarized sunglasses and so can probably appreciate how light can pass through two pieces of polarized film as long as they’re polarized in the same direction but is blocked if you rotate one of the films through 90 degrees. So our LCD sandwich would be opaque if it were not for the liquid crystal’s ability to change the polarization of light. The tiny furrows in the glass filters force the molecules of the liquid crystal to arrange themselves in a twisted pattern, so that they twist the polarization of the light that passes through them—allowing it to pass straight through the whole sandwich. But when an electrical voltage is applied to the liquid crystal layer, it overrides the twisted structure of the liquid crystal and forces the molecules to line up with each other. In this situation, the polarized light no longer twists as it is passed through the liquid crystal, so the sandwich becomes opaque.
Very large liquid crystal sandwiches do exist (Prada’s Manhattan store, for instance, has liquid crystal doors on its fitting rooms that change from transparent to opaque at the press of a button), but a practical display is made up of much smaller “picture elements” or pixels—hundreds of thousands of them—every one of which has to be supplied with its own individually controlled voltage. In a color display, each pixel is subdivided into red, green, and blue sub-pixels, whose colors are mixed together by the human eye to produce what we see as a colored picture.
Controlling such huge numbers of pixels and subpixels would be completely impossible if screen makers were still using the grids of wires that controlled the simple LCDs of the ‘70s. What made modern displays possible was a development known as Thin Film Transistors (TFTs) in which thousands of tiny transistors can be built into a thin transparent film and used as miniature switches to control the individual pixels of the display. It’s thanks to TFTs that we saw a sudden shift from the crude, blocky, monochrome displays of the ‘70s and ‘80s to the much higher-definition color displays of the late ‘90s.
Of course, progress didn’t end with the TFT. There were issues with the early color displays, in particular, because although they looked wonderful in showrooms and boat-show tents, they became almost invisible in sunlight. They simply were not bright enough to be seen by eyes that had adapted to the brightness all around them.
There are two ways of dealing with this problem. One is to try to overpower the sun by installing more powerful backlights; the other is to adopt the judo principle of using your opponent’s strength against him—in this case by using a mirror behind the display instead of a backlight. The brighter the light falling on the front of the display, the more light is reflected back through the display from the mirror. The main snag with this, of course, is that it doesn’t work at night. The compromise solution is known as a transflective display. It combines some of the properties of a reflective display (one with a mirror backing) with some of the properties of a transmissive display (one with a backlight) but no mirror. Transflective displays are so clearly the best choice for equipment that has to be used in a wide range of lighting conditions that they have become virtually standard for marine displays even though they tend to look slightly “muddy” alongside office PC displays in the relatively subdued lighting of a showroom.
Even the best transflective display cannot reflect all the light that falls on it, so it still needs backlighting, even in daylight. And in bright sunlight, it needs all the backlighting it can get. And it’s in backlighting that the latest big development in display technology is happening: the shift from Cold Cathode Fluorescent Lighting (CCFL) backlighting to light-emitting diodes (LEDs). CCFL elements look very much like miniature versions of ordinary fluorescent tubes. They work in almost exactly the same way, and have served us reasonably well for years. But they are old technology. There are limits to how bright they can be made without producing too much heat, limits to how dim you can make them before they go out altogether, and environmental concerns about some of the materials used to produce them. LEDs on the other hand, are the bright future, a developing technology that’s controllable, ethical, and economical. And that’s why your next display will very likely be the most readable one you’ve ever owned by far.
Want to know how to decipher the most common terms that manufacturers use to describe their LCDs? Check out the Jargon Buster.
This article originally appeared in the January 2011 issue of Power & Motoryacht magazine.